The most common instrument for determining the linear position of an object is probably the Linear Variable Differential Transformer, or LVDT. An LVDT is comprised of a primary winding, two secondary windings, and a movable magnetic rod that is attached in some manner to an object whose position is being determined. With an AC signal applied to the primary winding the magnetic coupling between the primary winding and the secondary windings varies according to the position of the magnetic rod, which in turn varies according to the position of the object. As the magnetic coupling changes the relationship between the outputs of the secondary windings changes. That relationship is used to determine the position of the object. LVDTs are capable of impressive accuracy, achieving sub-millimeter accuracy in some applications.
However, LVDTs tend to be relatively large, because an LVDT needs to be about twice as long as the range of motion to be measured, heavy, since a magnetic rod is required, difficult and expensive to manufacture, since three coils must be carefully wound on a core, and difficult to use, since electrical connections are required for each of the coils and since the relationship between the outputs of the secondary windings and the position of the object is not simple.
Various alternatives to the linear voltage differential transformer are known in the prior art. For example, U.S. Pat. No. 4,667,158, which issued on May 19, 1987 to Robert W. Redlich and which is entitled, "Linear Position Transducer And Signal Processor" discloses an inductive linear displacement transducer and an associated signal processor. As understood, U.S. Pat. No. 4,667,158, teaches a hollow, electrically insulated bobbin that is wound with wire to form an inductor. Partially inserted into the bobbin is a movable, conductive, but non-magnetic rod that has one end attached to an object whose position is being sensed and another end within the inductor. Surrounding the bobbin is a conductive shield that tends to confine the magnetic flux produced by the inductor inside the sensor. A magnetic, insulative layer is disposed between the inductor and the conductive shield. In operation an AC source excites a bridge circuit which has the inductor as one of its branches. Because of the skin effect, the magnetic flux within the bobbin, and thus the inductance of the inductor, depends upon the degree of insertion of the core into the bobbin. Since the output of the bridge depends upon the inductor's inductance, it is an indication of the insertion of the rod into the bobbin.
Various improvements to U.S. Pat. No. 4,667,158 have been made. For example, U.S. Pat. No. 4,926,123, which issued on May 15, 1990 to Robert W. Redlich and which is entitled "Precision Variable Pitch Compensation Winding for Displacement Transducer," teaches a compensating winding; and U.S. Pat. No. 5,115,193, which issued on May 19, 1992 to Bean et al., and entitled, "Inductive Displacement Transducer and Temperature-Compensating Signal Processor" teaches a method of temperature compensating linear positions sensors similar to that disclosed in U.S. Pat. No. 4,667,158. However, even with these improvements linear position sensing systems similar to that of U.S. Pat. No. 4,667,158 may not be optimal.
Wound inductors have been used in other measurement systems. For example, K. Lindstrom, H. Kjellander, and C. Johnson in "A New Instrument for the Measurement of Liquid Level," The Review of Scientific Instruments, pages 1083-87, Volume 41, number 7, July 1970 describe a liquid level monitoring instrument in which a transmission line having a helical wound conductor is used with time-domain reflectometry to determine the level of a liquid in a tank. Time-domain reflectometry is a measurement technique in which the time between the application of a pulse on a transmission line and the appearance of either that pulse or its reflection is used to determine some property of a system, typically the length of the transmission line or the location of an open, short, or connection on that line. It should be noted that time domain reflectometry also can be used to measure other properties, such as soil moisture or the permittivity of materials. In Lindstrom et al. electromagnetic pulses are impressed on their transmission line such that a liquid whose level is being measured can become between the helically wound conductor and an outer shield. The time required for a pulse to be impressed on the transmission line, to travel down that line, to reflect off of an interface between the liquid and a gas which fills the remainder of the line, and to return to the point of impression is used to determine the position of the liquid-gas interface.
Helically wound transmission lines are slow wave transmission lines. A slow wave transmission line is one having an effective speed of pulse propagation that is significantly less than the speed of pulse propagation in free space. By effective speed of propagation it is meant the axial speed of propagation (pulses in helical transmission lines spiral at high velocities, but axially propagate at a much slower speed). Slow wave transmission lines are advantageous since they change the problem of determining time intervals that result from pulses traveling at about 300,000,000 meters per second to the far simpler problem of dealing with time intervals that result from a pulse that travels much slower.
While slow wave transmission lines visually resemble the helical inductor/outer conductor of U.S. Pat. Nos. 4,667,158; 4,926,123; and 5,115,193, helically wound transmission lines operate differently. Input signals are not applied across the inductor, they are applied between the helically wound inductor and the ground conductor. Helically wound transmission lines are two port devices (even though an instrument according to the principles of the present invention might use only one port) that transmit signals from one location to another.
Lindstrom et al. also teach the use of sing-around. Sing-around is an advantageous technique that provides a simple, reliable, low cost method of converting pulse transient times (delays) into oscillations that have periods and frequencies which are easily measured. In Lindstrom et al. a pulse is impressed at the proximal end of the helically wound transmission line, that pulse travels along that line until it reflects from a material interface, the reflection travels back to the proximal end where the reflection is sensed and used to trigger another transmission line pulse. This process repeats, producing pulses at a frequency that depends upon the position of the material interface.
While helically wound transmission lines are advantageous in that they are low cost, rugged, and reliable it seems that, except for delay lines, they have been little used. Specifically, no prior use of a helically wound transmission line in a linear position sensor is known to the inventor. While this be an oversight, it seems that others have not fully appreciated helically wound transmission lines. Because of their low cost and general simplicity, time domain reflectometry linear position sensing, particularly those that use helically wound transmission lines, would be beneficial.